Functional Microstructures by Self-Assembly - Molecular Self-Assembly: Advances and Applications - page 401

12.3 Microscale Self-Assembly

The basic principle of microscale self-assembly is energy
minimization, which allows a large number of parts to be handled
in parallel. Typically, microscale self-assembly requires three basic
elements: (1) a substrate patterned with an array of energy traps,
(2) microscale component parts with a characteristic pattern, and
(3) an external agitation mechanism to assemble the substrate and
components together. The essence of a self-assembly procedure is
as follows: The substrate and the functional microcomponents are
first fabricated in large amounts, using modern microfabrication
techniques. After that, the components are introduced into a common
medium, in which the substrate is located. Then, the components
are agitated to recognize their energy traps, and attachment will be
accomplished by driving the system to its minimum energy state.
After the self-assembly, postprocessing is sometimes necessary to
hold the system for a longer period, which is needed in some actual
applications. Therefore, the binding mechanism, the microfabrication
process and the assembly procedure are the three key issues for the
successful development of micro self-assembly.
Different mechanisms can be applied to achieve energy
minimization for micro self-assembly, including gravitational force,
capillary force, and electrostatic and magnetic force [8]. Our group
has reported some work on 2D micro self-assembly mainly based
on gravity, fluid flow, and low melting alloys. This will be introduced
further in Sections 12.3.1 and 12.3.2 of this chapter.
Compared to 2D self-assembly as microcomponents are lying
in plane with the substrate surface, in 3D self-assembly process,
networks can be formed in either a parallel or serial fashion, which
poses more challenges to currently available microfabrication
techniques. Although 3D self-assembly is still in its infancy, our
group has reported some results, which will be introduced in Section
12.3.3 of this chapter.

12.3.1 Microscale 2D Self-Assembly on Template Guided

by Gravity and Fluid Flow

Gravity is a relatively weak force for microcomponents, but it
can be successfully used with fluid flow and shape recognition
in self-assembly processes. Fluid flow can be used to propel